A man has been charged with kidnapping a visiting University of Illinois scholar from China who authorities believe to be dead after she disappeared three weeks ago. A federal criminal complaint alleges the suspect's phone was used to visit an online forum in April called "Abduction 101."

Yingying Zhang, the 26-year-old daughter of a working-class factory driver from China, disappeared June 9, just weeks after arriving at the Urbana-Champaign campus in central Illinois where she was doing research in agricultural sciences and was expected to begin work on her doctorate in the fall. Friends and family described her as extremely bright, hardworking, caring and devoted to her parents.

Some 5,600 Chinese students are enrolled at the university — more than at any other college in the nation — and Zhang's disappearance fed anxieties of families of Chinese students studying in the U.S.

Federal authorities say Brendt Christensen, of Champaign, Illinois, was charged Friday, the same day he turned 28. A criminal complaint accuses him of abducting Zhang shortly after she stepped off a bus near the university campus. Video from cameras located around the area shows Zhang, who was on her way to sign a lease for an apartment, trying unsuccessfully to flag down another bus. Minutes later, she is seen getting into the front seat of a black Saturn Astra.

According to the 10-page affidavit filed in federal court by FBI Special Agent Anthony Manganaro, Christensen was under surveillance Thursday when agents overheard him explaining he had kidnapped Zhang. Authorities say agents believe Zhang is no longer alived based on that and other facts their investigation uncovered.

Asked if authorities had any leads as to where Zhang's body might be located, the spokesman for the FBI Springfield office, Bradley Ware, declined comment.

Christensen remained in custody pending an initial federal court appearance, set for Monday in Urbana.

An associate chancellor, Robin Kaler, said Saturday that Christensen earned a master's degree in physics from the University of Illinois in May, and that "his affiliation with the department ended that same month," though she didn't explain why. Christensen's LinkedIn profile posted online states he is a Ph.D. candidate in physics at the university and a graduate teaching assistant there since 2013. His profile also states that he graduated with bachelor's degrees in math and physics from the University of Wisconsin-Madison in 2013.

Illinois Chancellor Robert Jones said in a statement the campus community is saddened by the news that Zhang is believed dead.

"This is a senseless and devastating loss of a promising young woman and a member of our community," Jones said. "There is nothing we can do to ease the sadness or grief for her family and friends, but we can and we will come together to support them in any way we can in these difficult days ahead."

The federal charging document says one of the threads on the forum that Christensen's smartphone visited online in April — months before Zhang went missing — was entitled, "Perfect abduction fantasy." Another was about "planning a kidnapping."

According to Manganaro's affidavit, investigators determined there were 18 vehicles registered in Champaign County similar to the one Zhang got in.

One of those, belonging to Christensen, was first seen in an apartment complex parking lot on June 12 — days after Zhang went missing — and investigators questioned him. The affidavit stated that investigators noted Christensen couldn't recall what he was doing the day Zhang disappeared. They searched the vehicle but didn't remove anything.

Investigators later determined the car in the video had a sunroof and cracked hubcap, like Christensen's car, according to the affidavit. When investigators interviewed Christensen again, he acknowledged driving around the University of Illinois campus and giving a ride to an Asian woman who said she was late for an appointment.

Christensen said the woman panicked after he apparently made a wrong turn and he let her out in a residential area.

The court document states a search of Christensen's car indicates the area where Zhang was believed to have been sitting had been cleaned. It also states that "the other occupant" of Christensen's apartment agreed to let authorities search the home and take items, including Christensen's phone. It doesn't identify that person or their relationship.

Christensen was placed under continuous surveillance June 16. On Thursday he was captured on an audio recording explaining how he took Zhang to his apartment and held her against her will. The affidavit says the woman remains missing.

Zhang's father, Ronggao Zhang, traveled to Illinois from Nanping, China, to be closer to the search. On Thursday, he took part in a walk with students and university staff to the corner where his daughter was last seen.

Zhang graduated last year with a master's degree in environmental engineering from one of China's elite schools, Peking University Shenzhen Graduate School. She had been doing research on crop photosynthesis, which included using drones to study fields. Friends and family said she dreamed of one day landing a professorship and helping her parents financially.